Resolving NSTreeController's "Ambiguous use of 'children'" in Swift 3

I’m converting code for my first book to Swift 3. It uses Cocoa Bindings a lot, including NSTreeController.

Now the compiler changed; and one of the issues I faced was working with the NSTreeController.arrangedObjects. The compiler assumed a wrong type of the children property (see the docs) and reported “Ambiguous use of ‘children’”.

Thanks to StackOverflow, I found out that specific casting is supposed to help. The disambiguation didn’t work for me though.

To make this work, I had to move my custom tree node type to a different .swift source file. That type contains a children property to comply to NSTreeNode’s requirements. Thus the compiler assumed the unknown arrangedObjects as AnyObject cast really meant my own custom type. (That’s my explanation at least.)

So if your Swift 3 conversion doesn’t work as expected and your NSTreeController/NSArrayController/NSPageController is causing trouble …

move any type with a children property to a different file,

ensure you are specifying that you expect the children to be of type [NSTreeNode] – either through a cast (.children as [NSTreeNode]?), through specifying a helper function’s return type, or though specifying the expected variable type.

Help your compiler infer what’s going on in this still-very-weird casting situation.